The fight for a reactive site

A catalyst is a compound that speeds up a chemical reaction without being consumed itself. The proces of catalysis is one of the most important technologies in the modern world. Approximately 90% of all chemicals and materials around us is produced using catalysis. To get a better understanding of these industrial processes, it is important to investigate the exact role of the catalyst in the process, and the factors that can influence the outcome of the reaction. Therefore, it is necessary to first comprehend simple processes, such as the dissociation of hydrogen, the smallest molecule on Earth, on metal surfaces. In addition, this reaction is an elementary step in many industrial processes. My thesis describes the dissociation of hydrogen on bare and CO-precovered ruthenium, and on stepped platinum. These metals are important catalysts in e.g. ammonia synthesis (ruthenium) and fuel cells (platinum). I have discovered that ruthenium is a very good catalyst for hydrogen dissociation. The presence of CO, however, poisons the catalyst, and less hydrogen is able to dissociate. The presence of steps on a platinum surface increases the catalytic activity of the metal significantly.UBL - phd migration 201

Investigation of Active Catalysts at Work

Even after being in business for at least the last 100 years, research into the field of (heterogeneous) catalysis is still vibrant, both in academia and in industry. One of the reasons for this is that around 90% of all chemicals and materials used in everyday life are produced employing catalysis. In 2020, the global catalyst market size reached $35 billion, and it is still steadily increasing every year. Additionally, catalysts will be the driving force behind the transition toward sustainable energy. However, even after having been investigated for 100 years, we still have not reached the holy grail of developing catalysts from rational design instead of from trial-and-error. There are two main reasons for this, indicated by the two so-called "gaps" between (academic) research and actual catalysis. The first one is the "pressure gap", indicating the 13 orders of magnitude difference in pressure between the ultrahigh vacuum lab conditions and the atmospheric pressures (and higher) of industrial catalysis. The second one is the "materials gap", indicating the difference in complexity between single-crystal model catalysts of academic research and the real catalysts, consisting of metallic nanoparticles on supports, promoters, fillers, and binders. Although over the past decades significant efforts have been made in closing these gaps, many steps still have to be taken. In this Account, I will discuss the steps we have taken at Leiden University to further our fundamental understanding of heterogeneous catalysis at the (near-)atomic scale. I will focus on bridging the pressure gap, though we are also working on dosing the materials gap. Over the past years, we developed state-of-the-art equipment that is able to investigate the (near-)atomic-scale structure of the catalyst surface during the chemical reaction using several surface-science-based techniques such as scanning tunneling microscopy, atomic force microscopy, optical microscopy, and X-ray-based techniques (surface X-ray diffraction, grazing-incidence small-angle X-ray scattering, and X-ray reflectivity, in collaboration with ESRF). Simultaneously with imaging the surface, we can investigate the catalyst's performance via mass spectrometry, enabling us to link changes in the catalyst structure to its activity, selectivity, or stability. Although we are currently investigating many industrially relevant catalytic systems, I will here focus the discussion on the oxidation of platinum during, for example, CO and NO oxidation, the NO reduction reaction on platinum, and the growth of graphene on liquid (molten) copper. I will show that to be able to obtain the full picture of heterogeneous catalysis, the ability to investigate the catalyst at the (near-)atomic scale during the chemical reaction is a must.Catalysis and Surface Chemistr

ZnO(101¯0) is unstable in moderate pressures of water

A ZnO(10 (1) over bar0) single crystal was investigated using in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. In roughly 1 mbar water the surface roughens within minutes. Hereby, the formation of (0001)- or (000 (1) over bar)-type steps is favored over the formation of (1 (2) over bar 10)-type steps. The roughened surface is stable in ultra-high vacuum and does not exhibit a different amount of hydroxylation or adsorbed water compared to the as-prepared surface. The speed of the roughening is related to the total volume of water supplied to the surface rather than the water pressure.Catalysis and Surface Chemistr

Thermodynamic analysis of graphene CVD grown on liquid metal: Growth on liquid metallic gallium or solid gallium oxide skin?

A number of recent publications have reported liquid gallium to have an extraordinary catalytic activity for chemical vapor deposition (CVD) of graphene, enabling the growth of high-quality graphene on its surface even below 600 K. Our presented thermodynamic analysis however, indicates that during several of these experiments, an atomically thin gallium oxide layer should have covered the liquid gallium. This means that graphene should have actually grown on the solid oxide skin rather than on the liquid metal. This suggests a more complex mechanism for graphene growth on liquid gallium than what is currently considered in the community.Catalysis and Surface Chemistr